The invention relates to a centrifugal extractor comprising several superimposed stages in which two normally immiscible liquids are brought together in order to ensure the treatment of a first of the liquids by a second one of the liquids.
It should be noted that at least one of the two liquids brought together within the extractor can also contain suspended solid particles.
Centrifugal extractors are apparatuses producing an emulsion of a liquid to be treated and a treatment liquid, which are normally immiscible, with a view to aiding the treatment and then ensuring the separation of the two liquids by centrifuging. The liquid to be treated can in particular be constituted by a heavy aqueous phase, whereas the treatment liquid is a light, organic phase.
In practice, it is usually necessary to repeat these two operations a certain number of times in order to optimize the treatment. For this purpose, use is normally made of centrifugal extractors having several superimposed stages. In these centrifugal extractors the heavy aqueous phase to be treated circulates from top to bottom which is from the upper stage to the lower stage, whereas the light organic phase ensuring the treatment circulates from bottom to top which is from the lower stage to the upper stage of the apparatus.
Among the existing centrifugal extractors with superimposed stages, reference is made to the centrifugal extractors of type LX marketed by ROBATEL SLPI. Within a fixed enclosure, these centrifugal extractors comprise a bowl which can be rotated at high speed around a vertical axis and which defines internally several superimposed separating compartments. The rotary bowl is in turn placed coaxjelly around a fixed, central, tubular barrel or shaft, which extends towards the inside each of the separating compartments.
In the bottom of each of the separating compartments, the rotary bowl has a mixing chamber, which is separately penetrated by the heavy phase from the stage located immediately above, and the light phase from the stage located immediately below. A stirring disk, integral with the fixed central shaft or barrel, cooperates with the walls of the mixing chamber rotating at high speed in order to bring about an extremely fine emulsion of the light and heavy phases. Moreover, the assembly constituted by the mixing chamber, as well as the channels for introducing and discharging the two phases into and out of said chamber, together with the stirring disk, form a pump permitting the suction of the two phases from the adjacent stages.
The two phases of the emulsion created in this way are then separated by centrifugal force in the separating compartment of the corresponding stage. The relatively light phase is admitted into the mixing chamber of the stage immediately above it by a passage located in the vicinity of the fixed, central shaft. The relatively heavy phase redascends towards the mixing chamber of the stage immediately below it by channels located on the outer periphery of the rotary bowl.
Centrifugal extractors having several superimposed stages and constructed in this way are essentially satisfactory, but suffer from two significant disadvantages.
The first of these disadvantages results from the circulation of the heavy phase from a given stage to the stage immediately below it by channels formed in the rotary bowl on the periphery thereof. In view of the fact that the bowl rotates at very high speeds and which can, for example, reach approximately 20 to 30 m/s, the solid particles generally present in the heavy phase tend to agglomerate under the effect of the centrifugal force, which leads to clogging actions necessitating the stopping of the apparatus. Moreover, the high density of the agglomerated solid materials, due to the centrifugal force, makes the unclogging operations long and difficult.
A second disadvantage of centrifugal extractors with several superimposed stages of the type described hereinbefore is due to the air present within the apparatus. During the rotation of the bowl, the air is positioned between the fixed, central shaft and the liquid contained in the separating compartments. However, in view of the fact that the liquid is rotated at high speed by the rotary bowl, whereas the air layer in contact with the central shaft is virtually static, turbulence occurs and passes from the air into the liquid phases contained in the different separating compartments. This turbulence disturbs the separating efficiency obtained by centrifuging in the separating compartments, so that the efficiency of each stage is reduced.
FR-A-2 093 055 also discloses a centrifugal extractor having several juxtaposed stages, in which each of the extractor stages comprises a fixed casing or housing within the interior of which a bowl internally defining a separating compartment is rotated about a vertical shaft. The heavy and light phases are brought into the bottom of the fixed casing, where they are mixed by paddles integral with the rotary bowl, before rising in an axial passage located in the bottom thereof. This rising action is brought about by a pumping effect obtained by the cooperation of fixed blades carried by an axial rod integral with the casing and rotary blades installed in the axial passage of the rotary bowl traversed by said rod. The heavy and light phases are consequently introduced into a separating compartment formed within the bowl, so as to be separated by centrifuging. Overflows provided at the upper end of the separating compartment make it possible to pass the heavy and light phases respectively towards the two adjacent stages using vertical channels formed in the fixed casing of the stage in question.
This centrifugal extractor does not suffer from the disadvantages of that described hereinbefore. Thus, the path of the heavy phase between the adjacent stages takes place by gravity within the fixed casing, which considerably reduces clogging risks and facilitates unclogging when this proves necessary. Moreover, the air layer contained in the rotary bowl is in contact with the rotary shaft used for driving said bowl and with the liquid rotated in the latter. Consequently the turbulence caused in the liquid is reduced compared with the centrifugal extractor having several superimposed stages described hereinbefore. Therefore the efficiency of each individual stage is considerably improved.
However, the centrifugal extractor described in FR-A-2 093 055 has several juxtaposed stages, which requires a separate motor for rotating each of the extractor stages. This increases the overall dimensions, the prime cost and the operating costs of the apparatus. Moreover, this contributes to an increase in breakdown risks.
Moreover, the design of the centrifugal extractor described in FH-A-2 093 055 is such that the transposition of the solution proposed in the case of an extractor having several juxtaposed stages to an extractor having several superimposed stages is virtually impossible to carry out. In particular, the pumping of the mixed liquid, enabling it to rise within the rotary bowl into the separating compartment is obtained by a cooperation of fixed and rotary blades formed respectively on an axial rod integral with the bottom of the fixed casing and in an axial passage formed in the bowl around said rod. It is therefore impossible to downwardly extend the vertical driving shaft of the rotary bowl in order to rotate the rotary bowls of other stages located below the first stage.
The object of the present invention is to provide centrifugal extractor having several superimposed stages, whose original design makes it possible to take advantage of the advances provided by the centrifugal extractor with juxtaposed stages according to FR-A-2 093 055, while only requiring a single drive motor in the same way as is used in the centrifugal extractors having several superimposed stages of a conventional nature.